Nitrogen oxides (NOx) in the exhaust gases from combustion processes constitute a significant environmental hazard, and emissions of nitrogen oxides are subject to increasingly stringent restrictions. Diesel vehicles, especially commercial heavy-duty diesel vehicles, are subject to particularly stringent requirements. Under the Euro IV emission standard taking effect in Europe in 2005, the NOx emissions from heavy-duty diesel engines must not exceed 3.5 g/kWh, whereas the Euro V standard implemented in 2008 will limit the emissions to a maximum of 2 g/kWh.
The amount of nitrogen oxides in exhaust gases can be decreased by reducing these oxides. In principle, compounds containing ammonium nitrogen, such as ammonia, ammonium carbamate, ammonium carbonate, ammonium bicarbonate, ammonium formate, ammonium acetate, ammonium oxalate or ammonium hydroxide, can be used for this purpose.
A notable technical alternative in nitrogen oxide removal comprises the use of selective catalytic reduction (SCR techniques), in which nitrogen oxides are reduced into nitrogen and water by means of a catalyst and a reducing agent. Ammonia and urea have primarily served as reducing agents. The reducing temperature has usually been in the range 265-425° C., typically 300-400° C. For example V2O5-TiO2 catalyst has been used as the catalyst. SCR techniques have the additional benefit of allowing a decrease of the fuel consumption of the engine. Namely, this allows application of higher temperatures, involving more complete combustion but lower nitrogen oxide formation.
In the SCR techniques of vehicles, ammonia and ammonia water involve a problem in terms of operational safety. For this reason, urea, which is almost risk free, is preferably used. Urea is preferred for the additional reason that it contains a double amount of nitrogen per mole compared to the ammonia salts mentioned above. In fact, an aqueous solution of urea is a most appropriate nitrogen oxide remover. Urea is usually dissolved to form an approx. 32.5% by weight solution (e.g. AdBlue, a product name used by VDA), yielding sufficiently high solubility and the lowest resistance to temperature of the solution (−11° C.). Thus, European car manufacturers have chosen these techniques in order to meet the increasingly stringent requirements (Selective Catalytic Reduction, Final Report, ACEA 23, June 2003).
However, in Northern Europe and colder regions in general, the problem is that the outdoor temperature is frequently below −11° C., resulting in an obvious risk of freezing of the urea solution. The risk of freezing is reduced by installing various temperature sensors and resistors, with the primary purpose of preventing freezing or of remelting the frozen urea solution into liquid form. Thus, there have been suggestions to add various additives to a urea solution especially to achieve a lower freezing temperature.
The literature mentions the use of ethanol [F. F. Mao et al. “Urea-ethanol-water solution for diesel NOx control Using Urea”, 6th Diesel Engine Emissions Reduction (DEER) Workshop August 20-24] and propylene glycol [Lambert et al. “Application of Organic Freeze-Point Depressant in Aqueous Urea Solution: Effect of Nox Reduction”, SAE2003-01-0775] as an anti-freezing agent of an aqueous solution of urea used in the reduction of nitrogen oxides in exhaust gases from a diesel combustion process. In addition, it is generally known to use carboxylic acids for a lower freezing temperature of urea solutions. Such a use has been described e.g. in GB 111936. However, U.S. Pat. No. 6,387,336 states that e.g. ammonium formate is inappropriate for use with the aim to lower the freezing temperature of a urea solution used in an SCR-process, due especially to its corrosive properties.